Because it is sooo easy to lift something heavier than you should with the FEL, I try to keep my fronts at the max tire inflation limit. Especially since they do lose a little pressure over time. I worry a lot less about the rears.
Per the direct question: the mfg of the tractor will tell you the best compromise for their typical setup (just like car makers do). The tire ratings marked on the tire will tell you what it is rated for at a couple given pressures. Your range is between what the Tractor Mfg tells you and the Max limit for the tire. If you will be loading it heavy, then go towards the max. If you are not really stressing the tractor, go towards the tractor mfg spec. Other than that, don't worry about it too much.